GTS break barrel

[cooper_dan]

that looks excellent Dan.. very impressive. What power output and calibre is that ?

BTW On port size, 3.2 will still be quite restrictive, but still flow a good deal better than 3.0
if you are goign to max out your piston weight at more liek 265g, the 3.2 will still be on the tight side.

Thanks.
LGV is .177 and probably somewhere between 10.5-11 ft-lb, didn't chrono it.
Mine is also .177 (10" barrel). Think that was 9 ft-lb in that video. So it's a bit under. I'll do some more when it's at full power.

Thanks for advice re: TP. I'll certainly be trying 3.2mm soon!

[cooper_dan]

Bit of sneaky pre-work testing this morning (as little man decided 5am was a good time to wake us up).
Set the power to 10.5 ft-lb with a 250g active piston (the heaviest the active piston will currently go).
Then added in the dummy piston for total mass of 460g. Power dropped to 8.7 ft-lb
Then removed 50g off the active piston for a total mass of 410g. Power dropped to 7.1 ft-lb.

We roughly know that a 28mm pistons work best sub 300g, so those results don't seem to make sense. The 410g piston should make higher muzzle energy than the 460g.
But actually it does make sense, and I think it proves a point I was thinking about yesterday. With the dummy piston in place, there is a buffer seal (imagine the rear piston of the Diana twin piston rifles). Because the piston seal on the active piston compresses in use, the dummy piston seal/buffer must have an amount of pre-load on it, equal to that deformation. (If you read the Diana service manuals this is something like 0.3mm).
If the pre-load is too high, the active piston will not be able to complete it's full stroke. If the pre-load is too low, there will be excess load transmitted through the gears (causing accelerated wear) and you will lose the completely recoiless feel. Both pistons must come to the end of their stroke at exactly the same time.

In my testing this morning I have started off with a very high buffer pre-load which will be preventing the active piston making full stroke. However the 460g total piston mass is able to compress the buffer seal a bit more than the 410g mass, therefore completes a bit more stroke, and makes a bit more power.
Now I need to remove 0.1mm buffer pre-load at a time until I reach the correct amount. As I am actually using Diana piston seals, this is probably going to end up very similar to the 0.3mm that Diana specify!

Recently I seem to finding lots of reasons that other people don't already make this rifle
The real challenge will not be making it work, but will be keeping it as easy as possible to service. Still working on that!

[Shed tuner]

without knowing more about your mechanism, it's hard to opine. Other than that the dummy piston may be creating decelerative forces on the main piston, hence it needing more weight.

Drop us an email if you'd like me to have a look / think. Happy to do an NDA as I have no commercial interest in the airgun industry

[dvd]

Hi, I presume your system is similar to the Giss Diana.
0 If so, try and keep the backlash between the idler gears and the racks they mate with on the pistons to an absolute minimum or you will have a hard time getting both pistons to arrive simultaneously. To get an idea of this, use a very weak spring to load both pistons with the gears in place, then insert a rod through the transfer port and hold it against the piston seal while increasing the pressure inwards against the rear piston. If you can see it move before you feel the front piston move backwards, it means you have substantial backlash in the gears or their pinions or both.it all adds up.
I presume the dummy piston is bored, you don't want it to compress air at all, you just need it's mass and dimension.

If the front piston arrives before the rear one, you will get recoil and reduced efficiency due to moving a useless rear piston. If it's the other way round and the rear gets there first, you get lost volume and inefficiency. In both cases the gears will always be loaded trying to halt a piston that wants to move some more and that's bad news because they will break up eventually.
Think of the gears as idler gears, they should never be loaded except when the gun is cocked or to a lesser degree when the gun is assembled with the end block off.

[RustyBuzz]

All very interesting.

So is the dummy piston racked below the actual piston or is it in-line?

Richard

[cooper_dan]

Thanks for the offer shed tuner, but it was the buffer as expected. Narrowing down the buffer preload gave increasing fps on the lighter pistons, and almost no change on the heavier pistons.
Moving from 1.25mm to 0.25mm buffer preload gave an extra 50fps. It's quite sensitive but that's to be expected.

Dvd/rustybuzz. Diana giss is a good enough assumption without giving too much away. There are plenty of ways to go about it though. The mass doesn't have to equal the position mass if you gear up or gear down. You can play with ratios of mass and speed and still achieve the recoiless effect. Or the mass could be split into multiple parts. A collar around the outside of the active piston, and a heavy dynamic spring guide, linked together with cable or chain. This way the active piston can pass through the middle of the dummy to get more stroke and make 12 ft-lb.

Backlash is a funny one. With new parts (to spec) the backlash is around 0.3mm which seems to be OK. However if there any abuse loads (for example dry firing or firing without the buffer) the wear it causes on the gear mounting causes some slack. I've got some abused parts that have almost 1mm of backlash which is noticeable now.
I do have an anti-backlash setup just in case but was hoping to to need it. Time will tell!

[cooper_dan]

Put a different barrel on last night for a play.

10" .177 barrel = 9.7 ft-lb
16" .22 barrel = 10.4 ft-lb

I thought the combination of .22 and length would give me +1 ft-lb but not quite.
Breech seals are less than perfect so I've ordered some new ones. I'll try again with new seals and a bit more spring and see if the difference is the same.

Taking a lot of willpower to not just bosh the power up to 12. But softly softly catchee monkey

[Shed tuner]

sounds like your barrel pressure is dropping off pretty quickly for the longer .22 barrel to make so little difference. Why ? Air leak is possible.. or excessive piston bounce ? the air is either leaking, going back into the chamber, or the piston is being slowed towards the end of the stroke (no idea how). Or - for completeness - the swept volume is too low (but we know it's not), or there's a bunch of lost volume in the system (unlikely).

I wonder if a 16" .177 barrel would be virtually the same power as a 16" .22 barrel ? If so you just invented an interchangeable barrel rifle that doesn't have the classic power problem

[cooper_dan]

sounds like your barrel pressure is dropping off pretty quickly for the longer .22 barrel to make so little difference. Why ? Air leak is possible.. or excessive piston bounce ? the air is either leaking, going back into the chamber, or the piston is being slowed towards the end of the stroke (no idea how). Or - for completeness - the swept volume is too low (but we know it's not), or there's a bunch of lost volume in the system (unlikely).

I wonder if a 16" .177 barrel would be virtually the same power as a 16" .22 barrel ? If so you just invented an interchangeable barrel rifle that doesn't have the classic power problem

I'm going with an air leak (hence new breach seals). Piston seal is good (as far as I can test).
Got some new springs coming to test (reduce bounce a bit?).
Need to raid the gun cupboard to see what other barrels I've got. I know theres some meteors in there that might be 12"....

[cooper_dan]

I'm a bit in limbo with the current physical testing, as I wait for the updated pistons to be machined.
So I've revisited the CAD model of the trigger. After a couple of days running through the options it's now a bit simpler in design and 10mm shorter (length which comes directly off of the action).
All sears will now run on self lubricating bearings too, so no break in period or periodical oiling required. Sear contact area is very small, but in a very safe way (similar to old 10m match springers in a way).

Looking forward to getting some units made up now!

[cooper_dan]

Been a while since my last update.
Today I started a 2000 shot torture test of the new pistons and buffer mechanism. I've shoved a load of spring in, to the point where power and efficiency are below what they should be. Pistons are out of balance. Got the carbine barrel on so cocking is as hard as possible.
This should stress the gears, pistons, cocking mechanism etc. Basically if something is going to break, this test should find it!

Its horrid to shoot, and incredibly loud. I'll need ear defenders for any long sessions. And every shot brings a small tear to my eye. Years of my life in this design and I'm abusing it like this

[TonyL]

Good luck with the endurance testing, Dan.

[Shed tuner]

just to make life easier, the shorted barrel will only stress test your arm.. it will make no difference to the load on the gun, so you may as well fit the longer one and reduce the effort / noise

Good to see thorough testing though...

[cooper_dan]

just to make life easier, the shorted barrel will only stress test your arm.. it will make no difference to the load on the gun, so you may as well fit the longer one and reduce the effort / noise

Good to see thorough testing though...

Very good point. Longer barrel is the wrong calibre though.
Although the long barrel might put it the wrong side of legal, so would have to remove some preload...

[Shed tuner]

OK, stick a bit of pipe over the short barrel... reduced cocking effort for no power increase